1. Field of the Invention
This invention relates to a static frequency converter having an intermediate DC voltage link, a rectifier, and an inverter with main thyristors in a bridge circuit which are shunted antiparallel by bypass diodes in a bridge circuit. Each bridge circuit is shunted by a quenching circuit having a quenching thyristor, a commutating capacitor, and a commutating choke, and a common commutating capacitor is provided for every two bridge arms with a common main terminal and in which a device for limiting the current-dependent recharging is used.
2. Description of the Prior Art
Such a converter is described in German Pat. No. 2 510 357.
Depending on whether an uncontrolled or a controlled rectifier is employed, such converters have constant or variable intermediate-link voltages. They are used frequently, because of their simple concept, for supplying three-phase machines. To make up for commutation losses after each forced commutation, the commutating capacitor in these converters is recharged with energy from the load current stored in the commutation inductance and from the energy of the intermediate link. Thus, the ability to commutate at different intermediate-link voltages and different load currents is assured. For large load currents, however, and, in particular, in the event of overloading the converter, this current-dependent recharging can lead to a considerable increase of the voltage at the commutating capacitor, i.e., of the commutation voltage, and can thus bring about the danger of undue stress on the thyristors, diodes and capacitors. This danger exists particularly in the case of high power ratings of, for instance, 100 kVA and more.
According to the above-mentioned German Pat. No. 2 510 357, this danger can be countered by providing at least one damping capacitor, which is shunted across one bridge arm, for each two bridge arms having a common main terminal. Thereby, the time needed for load-current-dependent recharging of the commutating capacitor is reduced, for each commutation, by a time interval in which the damping capacitors participating in the commutation process are discharged or recharged. In this way, peaking of the commutation voltage is prevented, almost without losses. However, the method of the abovementioned patent can only be used in inverter circuits without blocking voltage.
The problem, then, is to provide a frequency converter of the type mentioned above in which unduly high commutation voltages can be avoided in inverter circuits with, as well as without, blocking voltage.